I have these days found the few Qt desktops discouraging Gtk+, first of all LXQt (in my all current non-Mint distro), the rest are, Budgie (switching to LXQt since their version 11 instead of GNOME) by Solus (an independent Linux), Hawaii by Liri (an Arch derivative), Lumina by TrueOS (an BSD system), but KDE honestly encourages Qt integrations alongside Gtk+ since its Plasma 4.
So will Linux Mint teams include few Qt blends like LXQt/Budgie, Hawaii, Lumina into your LMDE 3, and later Mint alpha (testing) distro?

So will Linux Mint teams include few Qt blends like LXQt/Budgie, Hawaii, Lumina into your LMDE 3...

I suspect the answer is no. LMDE receives less attention than normal Mint by the Mint devs, and Clem has indicated that he will not consider including more than Cinnamon and MATE in LMDE 3.

Mint alpha (testing) distro

There is not, and likely never will be, a Mint alpha or testing distro. It's simply too much work for the team and stretches their good efforts to improving Mint too thin. To quote Clem's reply to a user on the Linux Mint blog (the September 2017 edition):

Hi Peter, Debian Testing wouldn’t work for us. It’s a continuous flow of updates and that constantly breaks anything based on it. To give you an example we’d have to rebuild Cinnamon very often for it to work there and we would always be on the back foot.. reacting to breakages and conflicts after they happen. LMDE was based on Testing for a while in the past and it wasn’t viable.

"Once you can accept the universe as matter expanding into nothing that is something, wearing stripes with plaid comes easy."
- Albert Einstein